The present invention is directed generally to the area of test apparatus and more particularly to a moisture tester for determining the moisture content of materials such as grains.
Testing the constituent contents, and especially the moisture content of various agricultural grain products is well known. The prior art has devised a number of novel apparatus for more or less automating the moisture content determination procedure. In this regard, it is desirable in many applications to test the moisture content of a great number of samples of grains in a minimum amount of time.
One particularly successful prior art device is the "model 919".TM. moisture meter manufactured by Motomco, Inc. This moisture tester essentially determines the moisture content of a sample of material by determining its dielectric constant or capacitance. In this regard, a premeasured quantity of material is fed to a test cell which essentially comprises a capacitor. Since, the capacitance value of the test cell is determined by the dielectric properties of the sample of material contained therein, the capacitance of the material itself may be inferred from the observed change in capacitance over that of the empty test cell.
More specifically, the foregoing prior art device utilized a substitution method for obtaining the moisture content of the material. That is, the capacitance value of the test cell itself is initially determined, and thereafter a sample of material is introduced into the test cell, whereupon the capacitance thereof is again determined. Hence, the change in capacitance may be attributed to the sample of material, whereby its moisture content may be determined on the basis of known correlations between dielectric properties and moisture content for each particular material. These known correlations have been determined by empirical studies for each of a plurality of different grain materials.
Still more particularly, the prior art method utilized a balanced oscillator approach, whereby the test cell was included in the tank circuit of one oscillator, which was then "balanced" by means of a variable capacitor with a relatively stable reference oscillator. Accordingly, the circuit was once balanced with the empty test cell and again balanced with the sample of material to be tested residing in the test cell. Thereupon, the difference in capacitance value of the variable capacitor at each balance or "null point" could be taken as equal to the capacitance value and hence moisture content of the sample of material.
While the foregoing method and apparatus has found widespread commercial acceptance, there is room for further improvement. For example, with the "moisture meter" referred to above a meter was coupled to the two oscillators so as to give a minimum or "null" reading upon reaching a balanced condition. An indicator and a calibrated dial or scale was associated with the variable capacitor. Thus, a reading of the dial setting necessary to "null" the meter was noted, and a corresponding moisture content taken from a printed chart. A plurality of such charts were provided, one for each of a plurality of different grain products which might be tested. However, in the absence of a suitable chart, or in the case of a moisture content not within the range of adjustment of the variable capacitor, the unit would be incapable of obtaining the desired results.
Moreover, obtaining a reasonably accurate moisture determination, that is, within plus or minus one percent or better, requires a high degree of resolution in the dial or scale associated with the variable capacitor. In this regard, such resolution may be required down to the order of tenths of picofarads. The foregoing problems may be approached by the provision of a larger variable capacitor, however, such capacitors are not inherently linear. Moreover, with a larger capacitor, the exact null point or point of balance between the oscillators becomes increasingly less well defined, and hence proper observation of the meter becomes difficult. Also, the problem of mechanical variance or play in the indicator shaft, etc. intermediate the variable capacitor itself and the dial or scale may be a source of error. Moreover, resolution of the machine to a high degree of accuracy involved the provision of very fine dial divisions, often requiring the use of a magnifying glass to read and thus inviting further error.
Additionally, such accuracy requires a relatively stable and reliable circuit. In the foregoing prior art device a vacuum tube circuit was utilized in conjunction with a highly stable RC reference or calibration network for calibrating the instrument. While it is theoretically possible to replace vacuum tubes with solid-state devices such as FET's the substitution cannot be made without further circuit modifications. For example, while vacuum tube oscillators are reasonable linear with low harmonic distortion, solid-state or FET-based oscillators are generally noticeably more non-linear with significant harmonic distortion. In other respects, the output voltages developed in the pickup links in the vacuum-tube based oscillator circuits are effected by the transconductance of the tubes. This is of course eliminated with the substitution of solid-state devices.
Additionally, as previously noted, under certain conditions, for example, where moisture content and hence capacitance is relatively large, the "null" reading on the meter of the foregoing prior art device is not particularly well defined. Hence, some error may be introduced by inability of the operator to accurately determine by observation of the meter the exact point of balance between the oscillators.
As an additional matter, the mechanical aspects of the prior art device included a first tubular receptacle into which a premeasured quantity of the grain to be tested is introduced. This receptacle was then placed coaxially over the like-dimension tubular test cell and a pair of doors therein manually released to release the premeasured sample to the test cell. In order to minimize the handling of the sample of material and further insure accuracy, it is desirable to automate the foregoing procedure to some extent. In this regard, the prior art meter also required reference to temperature charts and separate measurement of the temperature of a sample to provide a correction factor for the moisture reading. In this regard, the first or "dial-reading" charts were established for a predetermined standard temperature, and hence variations in the actual temperature of the grain required reference to yet a further temperature correction chart. It is also desirable to automate this procedure.